1. Field of the Invention
The present invention relates to a flow sensor for measuring a fluid flow rate by using a thermoresistor (heating resistor).
2. Related Background Art
It has been conventional to use a flow sensor to the type in which a flow rate is measured by detecting the thermal equilibrium state of a bridge circuit including a thermoresistor (heating resistor) disposed in the fluid concerned. A conventional air flow sensor that utilizes a platinum wire as a heating resistor will be described hereinbelow.
FIG. 1(a) is a vertical side sectional view showing the structure of a thermal air flow sensor which utilizes a platinum wire as a heating resistor. FIG. 1(b) shows a front view of the structure shown in FIG. 1(a). In FIGS. 1(a) and 1(b), a measuring tube passageway 2 is supported by a supporting member 3 at a predetermined position in a housing 1, which serves as a main passageway for a fluid. A plurality of hot wire supporting members 4 are provided on the inner surface of the tube passageway 2. A hot wire R.sub.H is reeved through the hot wire supporting members 4 in the plane which is normal to the flow of air.
An air temperature sensor R.sub.c is also arranged in the measuring tube passageway 2. Electrically connecting lead wires of the hot wire R.sub.H and air temperature sensor R.sub.c are lead to the inside of a control circuit setting portion 5 provided on the outer periphery of the housing 1 through holes (not shown) formed in the housing 1, tube passageway 2, and supporting member 3 and are connected to a control circuit provided in the setting portion 5. Protecting nets 6a and 6b are attached to the opening portions on both sides of the housing 1.
FIG. 2 is a diagram showing a bridge circuit including the hot wire R.sub.H and air temperature sensor R.sub.c and a temperature control circuit 10 adapted to control temperature so that the bridge circuit maintains a thermal equilibrium state. The bridge circuit comprises resistors R.sub.1 and R.sub.2, hot wire R.sub.H, and air temperature sensor R.sub.c. Both input terminals of a differential amplifier 101 are connected to connecting points b and f of the bridge circuit. An output of the differential amplifier 101 is connected to a base of a transistor 102. An emitter of the transistor 102 is connected to one end a of the bridge circuit and a collector is connected to a positive polarity terminal of a DC power source 103.
The operation will now be briefly explained. Since the operation of a temperature control circuit is well known, detailed description thereof is omitted here. A simple explanation of the operation will be given for better understanding. When the voltages at the connecting points b and f are equal, the temperature control circuit reaches an equilibrium state. At this time, a current I.sub.H corresponding to the flow rate flows through the hot wire R.sub.H. A voltage V.sub.H at the connecting point b is expressed by V.sub.H =I.sub.H.R.sub.2 and this voltage is used as a flow rate signal.
In general, in order to correct any variation in measurement resulting from variations in the resistance values and resistance temperature coefficients of the hot wire R.sub.H and air temperature sensor R.sub.c or the resistance values of the resistors R.sub.1 and R.sub.2, the detection flow rate characteristic is changed in parallel by adjusting the resistance value of the resistor R.sub.1 so that a detection output value at a predetermined flow rate (ordinarily, a relatively low flow rate) is adjusted to an objective value.
FIG. 3 is a detection flow rate characteristic diagram for explaining the foregoing correction. The resistance value of the resistor R.sub.1 is adjusted so that a characteristic curve a before adjustment by the resistor R.sub.1 will lie within a given objective value range x at a predetermined flow rate Q.sub.1.
In the thermal flow sensor including the temperature control circuit 10 mentioned above the resistance value of the resistor R.sub.1 is adjusted (as shown in FIG. 3, the detecting characteristic is adjusted by changing the detection flow rate characteristic in parallel) in order to improve the measuring accuracy. However, it is impossible to adjust the gradient of the flow rate characteristic (flow rate dependency of the deviation from the center value of the detecting characteristic at each flow rate hereinafter referred to as a characteristic gradient) which is mainly based on structural and dimensional variations such as variations in the dimensions of the housing 1 and measuring tube passageway 2, variations in their relative positions changes in alignment of the center axis of the tube passageway 2 with respect to flow direction, variation of the reeving position of the hot wire R.sub.H, and the like. Measuring accuracy is not improved at flow rates other than the adjustment flow rate point Q.sub.1. This is particularly true at the flow rate which is largely deviated from the adjustment flow rate point Q.sub.1. There are the drawbacks as mentioned above.
When the resistance value of the resistor R.sub.1 is again adjusted in order to adjust the detection output at a flow rate point other than the adjustment flow rate point Q.sub.1, the detection output at the flow rate point Q.sub.1 also changes, so that the detecting accuracy cannot be improved at all of the flow rates.